1. Field of the Invention
The present invention relates generally to systems and methods of tuning an RF generator, and more particularly, to methods and systems for automatically tuning an RF generator for a substrate cleaning system.
2. Description of the Related Art
The use of acoustic energy is a highly advanced, non-contact, cleaning technology for removing small-particles from substrates such as semiconductor wafers in various states of fabrication, flat panel displays, micro-electro-mechanical systems (MEMS), micro-opto-electro-mechanical systems (MOEMS), and the like. The cleaning process typically involves the propagation of acoustic energy through a liquid medium to remove particles from, and clean, a surface of a substrate. The megasonic energy is typically propagated in a frequency range of about 700 kHz (0.7 Megahertz (MHz)) to about 1.0 MHz, inclusive. The liquid medium can be deionized water or any one or more of several substrate cleaning chemicals and combinations thereof. The propagation of acoustic energy through a liquid medium achieves non-contact substrate cleaning chiefly through the formation and collapse of bubbles from dissolved gases in the liquid medium, herein referred to as cavitation, microstreaming, and chemical reaction enhancement when chemicals are used as the liquid medium through improved mass transport, or providing activation energy to facilitate the chemical reactions.
FIG. 1A is a diagram of a typical batch substrate cleaning system 10. FIG. 1B is a top view of the batch substrate cleaning system 10. A tank 11 is filled with a cleaning solution 16 such as deionized water or other substrate cleaning chemicals. A substrate carrier 12, typically a cassette of substrates, holds a batch of substrates 14 to be cleaned. One or more transducers 18A, 18B, 18C generate the emitted acoustic energy 15 that is propagated through the cleaning solution 16. The relative location and distance between the substrates 14 and the transducers 18A, 18B and 18C are typically approximately constant from one batch of substrates 14 to another through use of locating fixtures 19A, 19B that contact and locate the carrier 12.
The emitted energy 15, with or without appropriate chemistry to control particle re-adhesion, achieves substrate cleaning through cavitation, acoustic streaming, and enhanced mass transport if cleaning chemicals are used. A batch substrate cleaning process typically requires lengthy processing times, and also can consume excessive volumes of cleaning chemicals 16. Additionally, consistency and substrate-to-substrate control are difficult to achieve. Such conditions as “shadowing” and “hot spots” are common in batch, and other, substrate megasonic processes. Shadowing occurs due to reflection and/or constructive and destructive interference of emitted energy 15, and is compounded with the additional substrate surface area of multiple substrates 14, walls of the process tank etc. The occurrence of hot spots, primarily the result of constructive interference due to the use of multiple transducers and to reflection, can also increase with additional multiple-substrate surface areas. These issues problems are typically addressed by depending on the averaging effects of the multiple reflections of the acoustic energy on the substrate, which can lead to a lower average power to the substrate surfaces. To compensate for the lower average power, and provide effective cleaning and particle removal, power to the transducers is increased, thereby increasing the emitted energy 15 and increasing cavitation and acoustic streaming, which thereby increases the cleaning effectiveness. Additionally, pulsing the multiple transducer arrays 18A, 18B and 18C is used (i.e. providing a duty cycle such as turning the transducers on for 20 ms, and then off for 10 ms. The transducers 18A, 18B and 18C can also be operated out of phase (e.g., activated sequentially) to reduce compound reflections and interference.
FIG. 1C is a prior art, schematic 30 of an RF supply to supply one or more of the transducers 18A, 18B, 18C. An adjustable voltage controlled oscillator (VCO) 32 outputs a signal 33, at a selected frequency, to an RF generator 34. The RF generator 34 amplifies the signal 33 to produce a signal 35 with an increased power. The signal 35 is output to the transducer 18B. A power sensor 36 monitors the signal 35. The transducer 18B outputs emitted energy 15.
The precise impedance of the transducer 18B can vary depending on many variables such as the number, size and spacing of substrates 14 in the carrier 12 and the distance between the substrates 14 and the transducer 18B. The precise impedance of the transducer 18B can also vary as the transducer 18B ages through repeated usage. By way of example, if signals 33, 35 have a frequency of about 1 MHz, the wavelength is about 1.5 mm (0.060 inches) in a deionized water medium such as the cleaning solution 16. As a result, referring again to FIG. 1A, if the location of the substrates 14 and carrier 12 is off by as little as about 0.5 mm (0.020 inches) or even less, the impedance of the transducer 18B can vary substantially.
Adjusting the frequency of the VCO can adjust the impedance of the transducer 18B by varying the frequency and therefore the wavelength of the signals 33, 35 and the emitted energy 15. Typically, a carrier 12 that is loaded with substrates 14 is placed in the tank 11 and the VCO 32 is adjusted to change the frequency of the signals 33, 35 and the emitted energy 15 until the impedance of the transducer 18B is matched, as indicated by a minimum value of a reflected signal 38 that is detected by the power meter 36. Once the VCO 32 has been adjusted to achieve the minimum reflected signal 38, the VCO 32 is typically not adjusted again unless significant repairs or maintenance are performed on the substrate cleaning system 10.
When the transducer 18B impedance is not matched, a portion of the emitted energy 17 (i.e., waves) emitted from the transducer 18B is reflected back toward the transducer 18B. On the surface of the transducer 18B, the reflected energy 17 can interfere with the emitted energy 15 causing constructive and destructive interference. The destructive interference reduces the effective cleaning power of the emitted energy 15 because a portion of the emitted energy 15 is effectively cancelled out by the reflected energy 17. As a result, the RF generator 34 efficiency is reduced.
The constructive interference can cause excess energy that can cause hot spots on the surfaces of the substrates 14 being cleaned. The hot spots can exceed an energy threshold of the substrates 14 and can damage the substrates 14. FIG. 1D is a typical transducer 18B. FIG. 1E is a graph 100 of the energy distribution across the transducer 18B. Curve 102 is a curve of the energy emitted across the transducer 18B in the x-axis. Curve 104 is a curve of the energy emitted across the transducer 18B in the y-axis. Curve 120 is a curve of the composite energy emitted across the transducer 18B in both the x-axis and the y-axis. The composite energy emitted across the transducer 18B in both the x-axis and the y-axis typically can vary between curve 120 and curve 122 as the known variations (e.g., location of the substrates, aging of the transducer, and wobble of a rotating substrate relative to the transducer etc.) cause the impedance of the transducer 18B to vary. A threshold energy level T is the damage threshold to the substrate(s) 14. Typically, the maximum power of the RF signal 35 and the resulting emitted energy 15 output by the transducer 18B is reduced to a level such that the maximum constructive interference results in a peak magnitude (i.e., peaks in curve 120) of less than the energy threshold T of the substrates 14 so as to prevent damage to the substrate 14. However, the reduced power of the RF signal 35 and the emitted energy 15 increases the cleaning process time required to achieve the desired cleaning result. In some instances, the reduced power of the signal 35 and the emitted energy 15 is insufficient to remove the some of the targeted particles from the substrates 14. As shown, the effective emitted energy can vary to a much lower level (represented by valleys in curve 122) such that the effectiveness of the cleaning process is severely impacted because the effective energy is so low (about 3) and therefore results in an energy window that extends from about 3 to about 17 as shown on the energy scale.
The transducer 18B is typically a piezoelectric device such as a crystal. The constructive and destructive interference caused by the reflected energy 17 can also impart a force to the surface of the transducer 18B sufficient to cause the transducer 18B to produce a corresponding reflected signal 38. The power sensor 36 can detect the reflected signal 38 that is reflected from the transducer 18B toward the RF generator 34. The reflected signal 38 can constructively or destructively interfere with the signal 35 output from the RF generator 34 to further reduce the efficiency of the RF generator 34.
In view of the foregoing, there is a need for an improved megasonic cleaning system that provides increased efficiency of the RF generator and a reduced energy window of the emitted acoustic energy and reduces the probability of substrate damage.